As development of rotor spinning machines progresses, the goal is not only to improve the quality of the yarns produced, but above all to increase production capacity. A key factor in increasing production capacity is the rotary speed of the spinning rotor. For this reason, varied kinds of drives and bearings for spinning rotors have been developed, in order to reach rotary speeds of markedly over 100,000 rpm. Reducing the rotor diameter and mass and lowering friction losses enables not only greater rotary speed but also reduced energy consumption when driven.
In this respect, a shaftless spinning rotor, which is embodied as the rotor of an axial field motor, can be considered especially advantageous by providing a combined magnetic and gas bearing which assures relatively low friction losses.
Shaftless spinning rotors, as initially developed (as disclosed for example in German Patent Publication DE 24 37 667 B2), included a set of magnetic windings disposed along a conically extending circumferential surface at the periphery of a spinning rotor. In contrast, axial field motors have proven to be advantageous in respect to the simplicity of their structures and simplified mounting.
For example, German Patent Publication DE 42 07 673 C1 discloses an axial field motor having a guide magnet arrangement disposed at the center of the rotor and the stator, which generates axial and radial forces and provides a dependable guidance for the spinning rotor. The rotor and stator are kept spaced apart by means of an air or gas cushion emitted from air nozzles associated with the bearing arrangement to act in opposition to the attractive magnetic force. However, under extreme conditions producing, for example, an imbalance of the rotor, the guiding force of the guide magnet arrangement may be insufficient to maintain the rotor in axial alignment with the stator. Accordingly, such known spinning rotors may be surrounded by a guide ring at an annular gap to limit the amount of possible rotor deflection. This safety measure is necessary in order to limit extensive damage during such extreme cases. Since the air gap between the bearing surfaces is very small, i.e., usually less than 1 mm, damage to the very delicate nozzle outlets of the gas nozzles can still occur in case of dirt accumulation or heat expansion in the central area of the bearing. In particular, the air essentially flows radially through the air gap to the exterior of the spinning rotor, so that in the center area cooling by means of the outflowing gas is less and dirt cannot be conveyed out from this area as effectively. In addition, the gas cannot flow off straight because of the mentioned guide ring.